Bath system for galvanic deposition of metals

ABSTRACT

A bath system for galvanic deposition of metals includes a solution containing at least one metal, especially a precious metal and/or precious metal alloy in the form of a water-soluble salt, at least one water-soluble protein material or amino acid and/or at least one water-soluble sulfonic acid, at least one water-soluble nitro-containing substance, at least one water-soluble surface-active agent and at least one vitamin. The bath system galvanostatically applies high quality layers with uniform quality. The bath system can be kept free of harmful substances such as cyanides, sulfites and hard complexing agents.

FIELD OF THE INVENTION

The invention relates to a bath system for the galvanic deposition ofmetals onto a metal substrate.

BACKGROUND OF THE INVENTION

Galvanic baths for the deposition of precious metals and precious metalalloys have long been known for use in the field of decorative arts andalso in technical fields. The soluble precious metal compounds which areused predominantly are in the form of a cyanide compound (potassium goldcyanide, potassium silver cyanide), a sulfite base (gold sulfitecompound), or contain ammonium (palladium compounds). The complexing andstabilizing of the systems select either an excess of the salts builders(cyanide, sulfite), or ammonium as well as the so-called hard complexbuilders (derivatives of amino-or nitriloacetic acids etc.) orcombinations of these.

Systems which contain cyanide are generally known and appreciated fortheir stability even with complicated and aggravated conditions of use.However, the toxicity of cyanide compounds are considerable sources ofdanger. Since a considerable interest exists in the use of the dangerouscyanide, continuous efforts and experimentation have been carried out inthe area of precious metal baths.

The use of sulfite gold baths and thiosulfate in silver baths hasalready been described. These systems, however, involve greatdifficulties with respect to the stability of the bath. Sulfite incomplex solutions initiates automatic reduction of precious metals, suchas for example gold. Thiosulfate in silver baths on account of the lowercomplexing strength cannot prevent photochemical reactions of freesilver ions. The addition of complex builders to the bath, such asethylene diamine tetraacetic acid or nitrilotriacetic acid or theirderivatives, can indeed slow down the photochemical reaction process butcannot prevent it. Careful control of the working pH level is required,so that a remarkably higher outlay is required with the processing ofrinsing and flushing waters and applied baths. The use of a gold sulfitecomplex is disclosed in German Patent DE-OS-38 05 627, wherebyhealth-endangering pyridine compounds as well as the nerve poisonantimony are added to the bath as additional component parts.

All of the bath systems known at this time, and accessible to the expertwith knowledge in the art, which were and are operated using replacementmaterials instead of cyanide, have not come into technical use onaccount of the difficult handling involved with these baths. Thus WO92/07975 discloses the use of aminoacetic acid and special glycine ascomplex builders for palladium and/or silver. However, such a bathsolution, which is a costly metal deposition using the so-calledpotentiostatic deposition method, is not comparable with thetraditionally used deposition method referred to as the galvanostaticmethod. Potentiostatic deposition demands unconditionally a depositionof anolyte and catholyte using a membrane, so that this known methodcannot be used for continuous industrial processes. In addition, the useof glycine alone in and of itself with a galvanostatic bath method wouldalso not lead to the desired results.

SUMMARY OF THE INVENTION

The invention is directed to aqueous bath systems for galvanicdeposition of metals, particularly of precious metals and their alloys,which in accordance with a stable working method are suitable both fordecorative and also for technical use. The process of the inventionfacilitates the application of high quality coatings which remain at thesame high quality during the galvanostatic working processes, and wherethe bath system is free of harmful materials, such as cyanides, sulfitesand hard complex builders. Furthermore, the number of salt buildersrequired for complexing of the individual metals is retained as low aspossible.

The disadvantages of the prior baths are overcome by the bath systemthat includes

at least one metal, particularly a precious metal and/or a preciousmetal alloy in the form of a water-soluble salt, and

at least one water-soluble protein, and/or

at least one water-soluble sulfonic acid, and

at least one water-soluble nitrO-containing material, and

at least one water-soluble surfactant, and

at least one vitamin.

DETAILED DESCRIPTION OF THE INVENTION

The bath of the invention is a highly effective, stable workingdeposition system, which can be maintained free of cyanides, sulfitesand hard complex builders. The bath system of the invention can be usedecologically. The bath system of the invention also meets the highestrequirements for environmental quality and waste disposal.

Therefore, the bath system of the invention facilitates the addition ofdifferent alloyed metals so that a broad spectrum of alloy compositionscan be attained. The bath system is characterized by the high quality ofthe deposition layers with simple maintenance of the bath. Furthermore,high current outputs are also possible, so that the amount of energyused can be reduced thereby reducing the operating costs. The bathsystem according to the invention is not noticeably dependent uponfluctuations of the component parts of the bath and the bath can easilybe filled according to the individual requirements by means of suitableadditives. Furthermore, the bath system has a long life.

Preferably, protein amino acids with their derivatives and their saltsare used as a supply for the protein materials, as well as additional oralternative sulfonic acids with their derivatives and their salts.

The aforementioned protein amino acids in the molecule in addition toone or more amino groups can also include one or more mercaptol groups.Examples of such protein amino acids are glycine, alanine, cysteine andmethionine.

The sulfonic acids can contain one or more alkyl groupings and/or one ormore amino groupings in the molecule. Examples of such sulfonic acidsare amidosulfonic acid, methansulfonic acid and ethane sulfonic acid.

For the stabilizing and complexing of the bath system, particularwater-soluble nitro-containing compounds have been proven favorable asacids and/or their derivatives and/or their salts. The citednitro-containing compounds can include at least one nitro group, onecarbonic acid radical and/or one sulfonic acid radical. Examples forsuch nitro-acids are 3-nitrophthalic acid, 4-nitrophthalic acid and/orm-nitrobenzene sulfonic acids.

Water-soluble nitrogen containing compounds such as acids and/or theirderivatives and/or their salts can be used for further stabilization ofthe system. The nitrogen containing compounds to be used contain atleast one amino group and/or one carbonic acid radical and/or sulfonicacid radical. Some examples of these nitrate compounds are nicotinicacid, its derivatives and/or salts and/or succinic acid, its derivativesand/or its salts, preferably its amino or sulfo compounds.

Materials that have proven particularly advantageous as surfactants arewater-soluble derivatives of succinic acid (Bernstein acid), namely itsester. Particularly, nicotinic acid amide is used as vitamin group, thenicotinic acid amide added to the vitamin B complex, particularly tovitamin B3.

Sulfone derivatives which are suitable for the system could be used aspolishing or glazing bath builder, which have one or two aromaticradicals on the sulfone group. The core carbon can be partiallysubstituted by metal, nitrate groups, or amino groups as well ashalogen.

For an expert in the art it is remarkable that the addition ofsurfactants as well as vitamins can maintain the otherwise known bathsfree of harm, particularly free of cyanide. It is also surprising thatthe surfactants and vitamins provide improved results, particularlyincreasing the deposition rates with comparable or lower. currentoutput. Therefore, improved surfaces are obtained as compared with theknown bath systems, even in relation to their glossiness.

A gold bath designed according to the invention for the deposition offine gold contains the gold, for example, as protein amino acidderivative of a class a) compound, preferably in the form of agold-cysteine complex, whereby the gold concentration is between 0.5 and30 g/l of the bath, preferably between 1 and 10 g/l of the bath. The pHlevel of the bath can be adjusted between 7 and 14, preferably between 8and 12, particularly between 9 and 10. The adjustment and stabilizationof the pH level can occur by means of a borate buffer, phosphate buffer,citrate buffer or some other buffer systems traditionally used ingalvanizing technology. The working temperature of such a bath isadjusted between 20° C. and 80° C., preferably between 40 and 60° C. Theusable cathodic current output density is between 0.1 A/dm² and 15A/dm², preferably between 0.5 A/dm² and 5 A/dm².

The characteristic gold bath according to the bath system of theinvention for the deposition of fine gold contains an excess of theprotein amino acid derivatives which are being used, and for thestabilization in addition to the aforementioned sulfonic acids caninclude one of the nitrogen-containing acids which is indicated and/orin addition a water-soluble nitrogen-containing compound. Thiswater-soluble nitrogen-containing compound, in addition to thecomplexing of the gold in a wide pH range, presumably if necessary canundertake the complexing of the alloy metals which are present. Theadditional water-soluble nitrogen-containing compound preferablyincludes at least one heterocyclic organic compound. This heterocyclicorganic compound can preferably consist of aromatic heterocycliccompounds. Amides of nicotinic acid have been shown to be particularlyadvantageous. As additional stabilizing medium, amides of succinic acidcan also be added.

A silver bath produced according to the invention for the deposition offine. silver includes the silver, for example, as sulfonate of the classb) compound, preferably in the form of silver methane sulfonate. Thesilver concentration is between 0.5 and 60 g/l of the bath, preferablybetween 2 and 40 g/l of the bath. For stabilization, the bath caninclude additional component portions of the sulfonate. For complexingof the free silver and for protection against photometric reaction, thebath of the invention includes at least one water-soluble protein aminoacid compound which is a class a) compound. The water-soluble proteinamino acid compound is present in excess of the stoichiometric ratio tothe silver.

For further stabilization of the bath, at least one additionalwater-soluble organic nitro-containing compound can be added to the bathsystem of the invention. This water-soluble nitro-containing compoundpreferably can undertake the complexing of alloy metals which arepresent in addition to the complexing of the silver in a wide pH rangeif it is called for. The added water-soluble nitro-containing compoundpreferably has at least one heterocyclic organic compound. Thisheterocyclic organic compound can preferably be an aromatic heterocycliccompound. The amides of nicotinic acid has been found to be particularlyadvantageous. For additional stabilization in terms of an amide, theamide of succinic acid can also be added.

The pH level of the bath can be adjusted to between 7 and 14, preferablybetween 8 and 12, particularly between 9 and 10. The adjustment andstabilizing of the pH level can be carried out with a borate buffer,phosphate buffer, citrate buffer or some other buffer used in thetraditional buffer systems in galvanizing technology. The workingtemperature of such a bath is between 10° C. and 50° C., preferablybetween 20° C. and 40° C. The usable cathodic current density can beadjusted to between 0.1 A/dm² and 15 A/dm², preferably between 0.5 A/dm²and 5 A/cm².

It is particularly advantageous that substrates of brass can be coateddirectly in the silver baths of the invention. The preliminary silvercoating of brass substrates in silver baths, which are required in theprior processes can be avoided. Alloy metals can be added to the bathsaccording to the invention for deposition of fine coatings of preciousmetals. All known metals of the periodic table can be used as alloymetals, which can be deposited together with the relevant precious metalout of an aqueous solution. Particularly, these are the aforementionedprecious metals: gold, silver, palladium and combinations of these sameprecious metals arranged one over the other. Preferably, copper, zinc,tin, iron and bismuth are suitable as alloy metals for gold. Tin andbismuth have proven particularly advantageous in their use as alloymetals for silver.

According to the baths of the invention using alloy depositions, theindividual alloy partner can be galvanically deposited by enrichment orby depletion in the baths into portions of between 1 and 100%.

Hereinafter relative to two exemplary embodiments, the aqueous systemhaving few or no harmful components for the galvanic deposition ofprecious metals and precious metal alloys is introduced.

The bath component parts in the following examples were dissolved in thegiven quantities and the solution filled with deionized water up to 1liter. Using the bath parameters given in the individual examples, testpieces were coated with the corresponding metal and/or metal alloy.

Bath 1: 2 g/l gold as gold-cysteine-complex 5 g/l methane sulfonic acidsolution (70%, neutralized with potassium hydroxide solution) 5 g/lcysteine 20 g/l borax 2 g/l 3-nitrophthalic acid 20 g/l succinic acidsulfamide 5 g/l nicotinic acid amide (vitamin) 1 ml/l Tegotain 485(commercial surfactant, 1% aqueous solution) 0.1 ml/l gloss builder(commercial sulfone derivative, 1% aqueous solution) temperature: 50-60°C. pH level: 9.5-10.5 current density: about 0.5 A/dm² anodes:platinum-coated titanium test piece: sections of silver-coated brasssheet, bent

The test pieces are coated over the entire surface. The color of themetal coating corresponds to the typical fine gold color.

Bath 2: 10 g/l silver as silver methane sulfonate 5 g/l methane sulfonicacid solution (70%, neutralized with potassium hydroxide solution) 5 g/lcysteine 20 g/l borax 2 g/l 2-nitrophthalic acid 25 g/l nicotinic acidamide (vitamin) 3 ml/l Tegotain 485 (commercial surfactant, 1% aqueoussolution) 0.1 ml/l gloss builder (commercial sulfone derivative, 1%aqueous solution) temperature: 25-30° C. pH level: 9.5-10.5 currentdensity: about 1 A/dm² anodes: fine silver test piece: sections of brasssheet, bent

The test pieces are coated over the entire surface. The color of themetallic coating corresponds to the typical fine silver color.

While various embodiments have been chosen to illustrate the invention,it will be understood by those skilled in the art that various changesand modifications can be made therein without departing from the scopeof the invention as defined in the appended claims.

What is claimed is:
 1. A bath for the galvanic deposition of metals,said bath containing: at least one metal salt or metal alloy salt in anaqueous solution, at least one component selected from the groupconsisting of a water-soluble protein material, a water-soluble sulfonicacid, and mixtures thereof, at least one water-soluble nitro-containingcompound, at least one water-soluble surfactant, and at least onevitamin.
 2. The bath of claim 1, wherein said protein material is anamino acid and is present as an acid, salt or derivative thereof, and ispresent in an amount of 0.1 to 150 g/l of the bath.
 3. The bath of claim1, where said sulfonic acid is present as an acid, acid derivative,salt, or mixture thereof in a an amount of 0.1 to 200 g/l of the bath.4. The bath of claim 3, a further comprising a water-soluble sulfonederivative as a gloss medium in a an amount of 0.001 to 5 g/l of thebath.
 5. The bath of claim 1, wherein said water-solublenitro-containing compound is present as an acid, acid derivative, saltor mixture thereof in an amount of 0.1 to 200 g/l of the bath.
 6. Thebath of claim 1, wherein water-soluble surfactant is present in anamount of 0.1 to 5 g/l of the bath.
 7. The bath of claim 1, wherein saidvitamin is nicotinic acid amide.
 8. The bath of claim 1, furthercomprising a buffering agent and where said bath has a pH of 7 to
 14. 9.The bath of claim 1, wherein said at least one metal is gold and wheresaid bath has a gold concentration of 0.5 to 30 g/l of the bath.
 10. Thebath of claim 9, wherein said bath has a gold concentration of 1 g/l to10 g/l.
 11. The bath of claim 1, wherein said at least one metal issilver and where said bath has a silver concentration of 0.5 to 60 g/lof the bath.
 12. The bath of claim 11, wherein said bath has a silverconcentration of 2 gl to 40 g/l.
 13. The bath of claim 1, wherein saidmetal salt is a precious metal salt.
 14. The bath of claim 1, whereinsaid metal alloy salt is a precious metal alloy salt.
 15. The bath ofclaim 1, wherein said metal is selected from the group consisting ofgold, silver, palladium, and mixtures thereof.
 16. The bath of claim 1,wherein said nitro-containing compound is selected from the groupconsisting of 3-nitrophthalic acid, 4-nitrophthalic acid, m-nitrobenzenesulfonic acid, and mixtures thereof.
 17. The bath of claim 1, whereinsaid water-soluble nitro-containing compound is present in an amount of0.1 g/l to 50 g/l.
 18. The bath of claim 1, wherein said bath has a pHof 8 to
 12. 19. The bath of claim 1, wherein said protein material is anamino acid selected from the group consisting of glycine, alanine,cysteine, and methionine.
 20. The bath of claim 1, wherein said vitaminis a Vitamin B complex.
 21. The bath of claim 1, wherein said bathfurther comprises borax.